eternal truth; unchanging principle
A somewhat formal or literary expression for a fundamental truth or principle that never changes.
万物は流転するが、不変の理も存在する。
All things are in flux, but there are also eternal truths.
Compound of 不変 (unchanging) + の (possessive particle) + 理 (principle, reason). The exact historical derivation is uncertain, but the phrase is used in philosophical and literary contexts.